Cyclophosphamide and Busulfan Followed by Donor Stem Cell Transplant in Treating Patients With Myelofibrosis, Acute Myeloid Leukemia, or Myelodysplastic Syndrome

Inclusion Criteria: * Idiopathic myelofibrosis (CIMF) * Myelofibrosis developing with polycythemia vera or essential thrombocythemia * Acute myeloid leukemia with or without antecedent hematologic disorder, at any disease stage (complete remission, minimal residual disease, or relapsed leukemia) * Myelodysplastic syndrome of any World Health Organization (WHO) or French-American-British (FAB) category, at any disease stage * Less than 61 years of age if transplanted from an unrelated donor, or less than 66 years of age if transplanted from a related donor * Receiving unmanipulated peripheral blood stem cells from an human leukocyte antigen (HLA)-identical or 1-allele-mismatched related or unrelated donor, or receiving G-CSF-stimulated bone marrow if co-enrolled on Fred Hutchinson Cancer Research Center (FHCRC) protocol 2250 * With a Karnofsky Performance score of \> 70% at the time of pre-transplant evaluation * Able to give informed consent (if \>= 18 years of age), or with a legal guardian capable of giving consent (if \< 18 years of age) * DONOR: HLA-identical or 1-allele-mismatched related or unrelated donors (by high resolution typing) * DONOR: Undergoing peripheral blood stem cell harvest or G-CSF-stimulated bone marrow harvest (bone marrow permitted only as part of FHCRC protocol 2250) * DONOR: In good general health, with a Karnofsky performance score of \> 80% * DONOR: Able to give informed consent (if \>= 18 years of age), or with a legal guardian able to give informed consent (if \< 18 years of age and donating for a related transplant) Exclusion Criteria: * Without an HLA-identical or 1-allele-mismatched related or unrelated donor * With human immunodeficiency virus (HIV) positivity or active infectious hepatitis * Receiving a medication known to strongly inhibit enzymes in the CYP450 pathway, and which, in the judgment of the consenting provider, cannot be safely discontinued for the duration of conditioning * Whose life expectancy is severely limited by diseases other than the hematologic disorder for which they are undergoing HCT (HCT-comorbidity index \[CI\] \> 3) * Women who are pregnant or lactating * With known hypersensitivity to BU or CY * With hepatic dysfunction as evidenced by total bilirubin or AST \> 2x the upper limit of normal, or evidence of synthetic dysfunction or cirrhosis * With impaired renal function, as evidenced by creatinine clearance \< 50% of expected, creatinine \> 2x the upper limit of normal, or dialysis dependence * With impaired pulmonary function, as evidenced by pO2 \< 70 mm Hg and diffusing capacity of carbon monoxide (DLCO) \< 70% predicted or by pO2 \< 80 mm Hg and DLCO \< 60%, or receiving continuous supplementary oxygen * With impaired cardiac function, as evidenced by ejection fraction \< 35% or active coronary artery disease * Unable to give informed consent * DONOR: Deemed unable to undergo stem cell collection, for any reason * DONOR: HIV-positive, or hepatitis B or C antigen-positive * DONOR: Women with a positive pregnancy test * DONOR: Unable to give informed consent (if \>= 18 years of age), or without a legal guardian able to give informed consent (if \<18 years of age)